Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. For instance, in a centripetal turbine the turbine housing defines an annular inlet passageway around the turbine wheel and a generally cylindrical axial outlet passageway extending from the turbine wheel. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the intake manifold of the engine, thereby increasing engine power.
Turbochargers incorporating wastegates are also well known. A wastegated turbocharger has a bypass passageway between the exhaust inlet and exhaust outlet portions of the turbine housing to enable control of the turbocharger boost pressure. A wastegate valve is located in the passageway and is controlled to open the passageway when the pressure level of the boost air increases to a predetermined level, thus allowing some of the exhaust gas to bypass the turbine wheel preventing the boost pressure from rising further. The wastegate valve is generally actuated by a pneumatic actuator operated by boost air pressure delivered by the compressor wheel. The position of the wastegate valve, and thus the amount of exhaust gas permitted to bypass the turbine wheel, is thus controlled in direct response to variations in the boost pressure.
A conventional pneumatic actuator comprises a spring loaded diaphragm or sliding seal housed within a canister (referred to as the wastegate actuator can) which is mounted to the compressor housing. The diaphragm seal acts on a connecting rod which actuates the wastegate valve assembly which is mounted in the turbine housing.
The actuator can is connected to the compressor outlet via a hose to deliver boost air to the can which acts on the diaphragm (or sliding seal) to oppose the spring bias. The spring is selected, and the actuator and wastegate valve initially set, so that under low boost conditions the wastegate valve remains closed. However, when the boost pressure reaches a predetermined maximum the diaphragm seal is moved against the action of the spring and operates to open the wastegate valve (via the connecting actuator rod) thereby allowing some exhaust gas to bypass the turbine wheel.
In conventional arrangements the wastegate valve is mounted on a valve stem which extends through the turbine housing and which is rotated to open and close the valve. Rotation of the valve stem is achieved by the reciprocal motion of the actuator rod via a lever arm which links the end of the actuator rod to the valve stem. To accommodate the motion of the actuator rod there is a pivotable joint between the lever arm and the actuator rod, the opposite end of the actuator lever being secured (typically by welding) to the end of the valve stem. For accurate operation of the actuator it is also important that the diaphragm seal maintains alignment within the canister, and thus that the rod maintains its alignment along the axis of the actuator can. It is therefore known to design the pivotal joint between the actuator rod and the lever arm to allow a slight amount of movement along the axis of the actuator lever to limit the tendency of the actuator rod to pulled off-line as it reciprocates.
The “lift off point”, i.e. that pressure at which the wastegate valve begins to open, is critical to operation of the wastegate and therefore must be very carefully set when the actuator and wastegate are assembled to the turbocharger. The precise actuator can pressure at which the diaphragm begins to move is dependent upon the preload of the spring used. Unfortunately, because tolerances to which springs can practically be manufactured mean that there can be variations in spring rate from one spring to the next, it is necessary to set the lift off point of each turbocharger individually.
One method of carrying out the initial set up of the conventional actuator assembly described above, is a process known as “weld to set”. The actuator can, actuating rod and actuator lever are pre-assembled, and mounted to the turbocharger. The wastegate valve is then clamped shut from within the turbine housing and the actuator can is pressurised to the desired lift off pressure. With the diaphragm, actuator rod and valve thus held in their respective relative positions immediately prior to lift off, the end of the actuator lever welded to the valve stem. Accordingly, any increase in the pressure supplied to the actuator above the predetermined lift off pressure will cause the valve to open.
A known alternative to the above is to use an adjustable length actuator rod, typically comprising a threaded rod and rod end. The set point is achieved by adjusting the length of the rod, either by turning the rod end or a nut captured in the rod end assembly.
European patent publication number 0 976 919 discloses a two-part actuator rod which overcomes many of the disadvantages of the conventional actuator rod described above. The two parts of the rod are connected via a spherical joint located towards the wastegate end of the rod which allows rotational freedom between the end of the rod connected to the actuator and the end of the rod connected to the actuator lever. This arrangement greatly simplifies initial set up by obviating the need to pre-assemble the lever to the actuator rod (the lever instead being pre-assembled to the wastegate valve stem) or the need to provide an adjustable length rod. However, both of the above actuator assemblies suffer the disadvantage that the respective pivotable joints add to the component cost and provide a site for wear to occur.